An active light emitting device used for a light emitting display may have a light emitting layer formed between two electrodes. The light emitting device can be classified as an inorganic light emitting device or an organic light emitting device depending on material. The light emitting device may further be classified as a passive matrix type or an active matrix type, depending on a driving mode of the light emitting layer.
FIG. 1 is a conventional light emitting device including a driver 10 formed on a fist transparent substrate 12, and a first electrode 16, a light emitting layer 18, and a second electrode 20 provided on the driver 10. In order to protect the device against moisture and oxygen, a getter unit 22 is formed on the second substrate 28, and the first and second substrates 12 and 28 are sealed by a sealant 26.
In the driver 10, the gate electrode 31 is formed on the first substrate 12, and a gate insulating film 32 is formed on the first substrate 12 to cover the gate electrode 31. An active layer 35 is formed of amorphous silicon (a-Si) or polycrystalline (poly) silicon (P—Si) on the gate insulating film 32, and an ohmic contact layer 34 is formed on the active layer 35. A source electrode 36 and a drain electrode 37 are formed on the active layer 35.
A protective film 38 can be selectively formed on the first substrate 12 comprising the source and drain electrodes 36 and 37. In detail, the drain electrode 37 electrically connects with the first electrode 16 through a contact hole which exposes a predetermined region of the drain electrode 37. The first electrode 16 electrically connects with a light emitting layer 18 as a pixel electrode.
Electrical current caused by driving will be described below with reference to FIG. 2.
When voltage is applied to the gate electrode 31, a channel is formed in the active layer 35 by the voltage of the gate electrode 31. Current (I) applied to the source electrode 36 flows to the drain electrode 37 along a low resistance layer formed in the channel. Since the active layer 35 is formed on amorphous silicon or poly silicon, the driver 10 has a characteristic in which a channel region formed by the gate electrode 31 has a relatively high resistance.
For description of resistance characteristic, it is assumed that resistance of the channel formed by the voltage of the gate electrode 31 is denoted by “R1.” Also, it can be appreciated that the following resistance characteristic is provided on the assumption that a resistance formed at a lower surface of the source electrode 36 is denoted by “R2,” and a resistance formed in the active layer 35 is denoted by “R3.”
When a voltage is applied to the gate electrode 31, the current density (I) between a source and a drain through the channel decreases due to the series resistance of R1+R2+R3. This means that the mobility between the source and the drain through the channel decreases. As a result, a response characteristic of an active current-driven organic light emitting device deteriorates.
Such a deterioration of the response of a current-driven display that uses a light emitting device such as an organic light emitting device or an inorganic light emitting device may impair the performance of the display.